What you were expecting to happen, and what actually happened?
I wanted to change my Guild Wars defense team for the next day. It accepted the change, displaying the new team set. However, when the next day started, it always had reset to the one assigned by default. I’ve had this happen to me several days in a row now.

What are the steps to make it happen again?
Go to Guild -> Battle -> Defense, click on the banner of the upcoming team, select any premade team from the pool. The defense tab will update to show the team has been set. Wait a day for the next round of battles to start. The defense team will have reset to the one initially assigned.

Same happens to me. I had different teams for each day set, but come to actual day it changes back to the same team for each day.
Can’t wait to not get points for using unique troops as it doesn’t allow me to use unique troops.

I finally made them stick some hours ago without reverting to one team across the board.
What Sudousai said but I went back to the map screen between setting each one :-

On the defence screen click on the “Day 1” banner. (Takes you to your teams list)
Click the banner of the team you want to select.
Back out of Wars to the map screen.
Do this for all 6 days.
Restart the game.